Load limit switch

ABSTRACT

A load limit switch for interposition between lifting machinery and a suspended cable including a housing connected to the lifting machinery, an inclined plane rigidly mounted in the housing, and a block slidably positioned on the inclined plane within the housing. The block includes means for suspending therefrom the cable for supporting weights, and a biasing spring is positioned between the housing and block, biasing the block with a preselected force at a position on the inclined plane. When the weight on a cable suspended from the block exceeds a predetermined level, the lateral component of force from the block overcomes the biasing spring force. The block translates on the inclined plane and activates a switch opening an electrical circuit which controls the lifting machinery.

United States Patent 1111 3,569,650

[72] Inventor Robert E. Chubbuck FOREIGN PATENTS g z g 3 Ave-1 Sam Fe 1,119,883 4/1956 France 200/85 [211 App! g? 8 Primary Examiner-David Smith,Jr. [22] d 17, 1969 Attorney-Townsend and Townsend Patented Mar. 9, 197] ABSTRACT: A load limit switch for interposition between [54] LOAD LIMIT SWITCH lifting maichinelry a susptlelnded cable incluging a housglg connecte to t e 1t1ng mac mery, an 1nc1ne pane r1g1 y I m Clams 5 Drawing Fm mounted in the housing, and a block slidably positioned on the [52] US. Cl. 200/85, indined plane within the housing The blok includes means 254/173 for suspending therefrom the cable for supporting weights, [5]] lrlLCl ..H0lh 13/16 and a biasing Spring is positionedbetween the housing and Field of Search 200/; k, bi i he block with a preselected force at a position 254/174 173; 192/150; 212/132, 39 on the inclined plane. When the weight on a cable suspended from the block exceeds a predetermined level, the lateral' [56] References Cited component of force from the block overcomes the biasing UNITED STATES PATENTS spring force. The block translates on the inclined plane and 2,636,953 4/1953 Hunt 200/85 activates a switch opening an electrical circuit which controls 3,262,581 7/1966 Linowiecki 200/85 the lifting machinery.

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INVENTOR ROBERT E. CHUBBUCK oh.) PM ATTORNEYS PATENTEU MAR 9 I971 SHEET 2 OF 3 IN F 3 ROBERT E. CY-I LT SCK MW ATTORNEYS PATENTED MAR 9|97| 3,569,650

sum 3 er 3 INVENTOR. ROBERT E. CHUBBUCK WNW ATTORNEYS LOAD LIMIT SWTTCH This invention relates to a new and improved load limit switch for protecting cranes, hoists, and other lifting devices from overloads.

it is an object of the present invention to provide a new and improved load limit switch for interposition between lifting machinery and a suspended cable from which weights are to be supported in order to shut off the lifting machinery when the weight lifted on the suspended cable exceeds a predetermined level. 7

Another object of the invention is to utilize only a small fraction of the force imparted to the suspended cable by supported weights to operate a load limit switch, thereby reducing hardware requirements and extending the life of the switch.

A further object of the invention is to provide a load limit switch which minimizes loss of head room at the point of connection between the lifting machinery and suspended cable.

in order to accomplish these results, the present invention contemplates a load limit switch for interposition between lifting machinery and a suspended cable'formed with a housing connected to the lifting machinery, an inclined plane rigidly mounted in the housing, and a block slidably positioned on the inclined plane within the housing. The block includes means for suspending therefrom the cable for supporting weights,

and a biasing spring is positioned between the housing and block, biasing the block with a preselected force at a position on the inclined plane. When the weight on a cable suspended from the block exceeds a predetermined level, the lateral component of force from the block overcomes the biasing spring force. The block translates on the inclined plane and activates a switch opening an electrical circuit which controls the lifting machinery.

Other objects, features and advantages of the present invention will become apparent in the following specification and accompanying drawings:

FIG. 1 is a side view in partial cross section of a load limit switch according to the present invention.

FIG. 2 is a plan view of the load limit switch with the top plate removed.

F16. 3 is a side view in partial cross section in the direction of the arrows on line 3-3 of FIG. of another load limit switch embodying the present invention with a weight less than the critical level supported on the cable.

HO. 4 is a side cross-sectional view of the load limit switch with a weight in excess of the critical level supported on the cable.

F IG. 5 is a plan view of the load limit switch as shown in F 1G. 3.

in the embodiment of the present invention shown in FIG. 1, a load limit switch is formed with a housing 11 having a top extension 12 bored for attachment or connection to lifting machinery such as cranes and hoists. Rigidly fixed to the housing 11 and mounted near the bottom of. the housing is a first block 13 beveled along its top surface to provide an inclined plane 14. The block 3 is formed with passageway 13a therethrough which is aligned with a hole 15 in the bottom of housing 11 in order to permit a cable 16 to pass through the hole 15 and block 13. Resting upon the inclined plane 14 of the first block 13 within housing 11 is a second block 17 beveled along its bottom edge to provide a surface complementary to the inclined plane 14. Block 17 is freely mounted within the housing to permit translation along the inclined plane 14 but is restrained to a reference position by a biasing spring 113 connected to the housing and abutting against a button 20 connected to block 17. The block 17 is similarly formed with a hollow core in order to receive the cable 16. Such a cable would normally be formed with an eye 21 for suspension of the cable over an anchor pin 22 which spans the hollow core of block 17 and is connected at the sides of the biock. The parts of the load limit switch described above can be formed of a suitable material such as cold-rolled steel except the block 13 which is eg bronze.

Mounted on the side of housing 11 opposite biasing spring 18 is an electric switch utilized to control an electric circuit which in turn controls the lifting machinery. Electric switch 25 is provided with a pushbutton 26 facing block 17 and normally biased as, for example, by a spring in an outward extending position. in its reference position on inclined plane 14, however, block 17 depresses pushbutton 26. Electric switch 25 can be constructed to that while pushbutton 26 is depressed, the electric circuit is closed permitting operation of the lifting machinery. The block 17 is maintained in its reference position as shown in FIG. 1 by means of the tension imparted by biasing spring 18. The biasing spring 18 is a strip of resilient metal having spring and force characteristics determined by considerations hereinafter described.

It is apparent from the construction heretofore described that only a fractional component of the force imparted to block 17 by weights on suspended cable 16 will be directed laterally against the biasing spring 18. Thus, because of the inclined plane action imparted by the beveled edge 14 and the positioning of spring 18, only the horizontal component of force transmitted by block 17 slidably mounted on the inclined plane is directed against the spring 18. Trigonometric analysis indicates that for an ideal frictionless inclined plane, the fraction of dead weight supported by cable 16 is directed against the biasing spring 18 is equal to sin 9 cos 9 where 173 is the angle of the beveled edge 14 with respect to the horizontal. If the inclined plane forms an angle of 45with respect to the horizontal, for ideal frictional surfaces, approximately one-half of the weight suspended on cable 16 would be transmitted against the biasing spring. In the example of the present invention described herein, the beveled edge 14 is formed to provide an angle of 15 with respect to the horizontal. For an ideal inclined plane at an angle of 15, approximately onequarter of the dead weight is deflected laterally against biasing spring. Of course, this laterally directed fractional component is reduced by frictional forces between the contacting surfaces.

By means of this arrangement, the routine operating stress on biasing spring 18 is significantly reduced. The tension and compressive strength of biasing spring 18 is fixed and adjusted to withstand forces imparted by block 17 due to weights on suspended cable 16 below a predetermined safe level for the lifting machinery with which the load limit switch is being used. When the operator of the lifting machinery attempts to lift by means of cable 16 a weight in excess of the predetermined safe level, biasing spring 18 is compressed permitting translation of block 17 away from electric switch 25. Pushbutton 26 therefore projects outwardly, opening the electrical circuit thereby shutting off the lifting machinery. The operator must thereafter reduce his load to that below the predetermined safe weight level before the block 17 returns to its reference position under the force of biasing spring 18 thereby depressing pushbutton 26 closing the electrical circuit.

Another load limit switch embodying the present invention is shown in FIGS. 3, 4 and 5. Referring to FIGS. 3 and 5, the load limit switch 30 is formed with a housing 31 including a top extension 32 bored for attachment to the lifting machinery. Housing 31 is formed by side members 33 and 34 and end members 35 and 36, and also includes the top plate. Rigidly mounted in the bottom of the housing and connected respectively to sides 33 and 34 of the housing are blocks 37 and 38. (Block 38 does not appear in the views shown.) The blocks 37 and 38 are provided with beveled stop surfaces 40 which form an inclined plane. The blocks 37 and 38 are spaced apart to permit passage therebetween of a cable. Positioned within the housing 31 and resting upon the inclined plane 40 formed by blocks 37 and 38 are blocks 41 and 42 respectively. Blocks 41 and 42 are spaced apart and rigidly connected to each other by bar 43 attached at one end of the blocks 41 and 42, and rod 45 generally centered between and passing through holes bored in the blocks 41 and 42. Cable 46 passing between blocks 37 and 38, and 41 and 42, respectively, can thereby be suspended from rod 45 by means of, for example, eye 47. The

blocks 41 and 42, rigidly connected together, are freely mounted in the housing for translation along the inclined plane 40. Blocks 41 and 42 are normally biased in a reference position against the end piece 35 of the housing by means of a coiled spring 50 which imparts a preselected force to block 42. The biasing spring 50 is mounted between the end member 36 of the housing and the bar 43 which rigidly connects blocks 41 and 42 together. A knob 51 with an externally threaded annular projecting flange 52 threadedly engages the surface of a hole 53 bored in the end of member 26 of the housing. The biasing spring 50 is retained within the annular projecting flange 52 and the spring tension and consequent force imparted to blocks 41 and 42 can be adjusted by turning the knob 51. The opposite end of spring 50 from annular flange 52 of knob 51 abuts against the connecting bar 43, thereby maintaining blocks 41 and 42 in a reference position against end member 35 when a weight level below a predetermined safety level is supported on a cable 46 suspended between blocks 41 and 42. The spring and force characteristics of biasing spring 50 are selected and adjusted so that critical depression of the spring will not occur until the load suspended from cable 46 exceeds a predetermined weight level.

Rigidly mounted in the housing below spring 50 is an electric switch 55 actuated by pushbutton 56. When the load on cable 46 exceeds a predetermined safety level, biasing spring 50 is compressed by the horizontal component of force transmitted by blocks 41 and 42 on the inclined plane 40: The electric switch 55 and pushbutton 56 are positioned so that one of the blocks 41 or 42 depresses pushbutton 56 upon translation of the blocks down the incline plane 40 under the force of a suspended load, as illustrated in FIG. 4. When pushbutton 56 is depressed, the bar 43 of block 42 bottoms against the endpiece 36 of housing 31 in order to protect the pushbutton and switch from damage when excessive weights are lifted. Electric switch 55 can be adjusted so that on depression of pushbutton 56 an electric circuit which controls the lifting machine is opened thereby shutting off the machinery. The lifting machinery thereby remains inoperative until the load on cable 46 is reduced below the predetermined safety level thereby permitting blocks 41 and 42 to return to a reference position under the stress of biasing spring 50. Alternatively, or in addition, the switch can be utilized to activate an alarm signal device. I

Because of the inclined plane 40, only a fractional horizontal component of the force imparted to block 42 by a suspended load is directed against spring 50, in the manner and according to the analysis set forth above. It is also apparent that by means of the invention described above, head space required by the load limit switch is minimized.

lclaim:

1. A load limit switch comprising: a housing; an inclined plane rigidly mounted in said housing; block means positioned on and slidable relative to said inclined plane and housing, said block means including means for suspending therefrom a cable for supporting weight; resilient bias means positioned between the housing and block means for biasing said block 'means with a preselected force at a position on the inclined plane; and switch means activated by said block means upon translation of the block means a predetermined distance along the inclined plane against said spring bias means under the force of weight above a predetermined amount supported on a cable suspended from said block means.

2. A load limit switch as set forth in claim 1 and including means for adjusting the resilient bias means for bearing said preselected force imparted to the block means.

3. A load limit switch as set forth in claim 1 wherein said switch means comprises an electric switch actuated by a pushbutton, said electric switch mounted in the housing at a position so that the pushbutton is depressed by the block means when the block means is at said reference position on the inclined plane.

4. A load limit switch as set forth in claim 1 wherein said switch means comprises an electric switch actuated by a pushbutton, said electric switch mounted in the housing m a position so that the pushbutton is depressed by said block means upon translation of said block means a predetermined distance along the inclined plane under the force of weight above a predetermined amount supported on a cable suspended by said bloclt means.

5. A load limit switch for interposition between lifting machinery and a suspended cable from which weights are to be supported comprising: a housing with means for connection to said lifting machinery; an inclined plane rigidly mounted in said housing; block means positioned on and slidable relative to said inclined plane and housing, said block means including means for suspending therefrom a cable for supporting weights; spring bias means positioned between the housing and block means for biasing said block means with a preselected force at a reference position on the inclined plane; switch means actuated by said block means upon translation of said block means a predetermined distance against said spring biased means along the inclined plane under the force of weight above a predetermined amount lifted on a cable in turn suspended from said block means; and circuit means controlled by said switch for shutting off the lifting machinery.

6. A load limit switch as set forth in claim 5 and including means for adjusting the spring bias means for varying the preselected force imparted to said block means.

7. A load limit switch comprising: a housing; means for suspending from said housing a cable with which weights are to be lifted; means mounted in said housing for directing a fraction of the force on the suspended cable due to lifted weights in a lateral direction within the housing; means in said housing slidable in response to said laterally directed fractional force; spring means mounted in said housing and biased to oppose with a predetermined opposing force motion of said slidable means due to said laterally directed fractional force; and switch means activated by said slidable means when said laterally directed fractional force exceeds said predetermined opposing force; said means mounted in said housing for laterally directing a fraction of the force on the suspended cable due to lifted weight comprising an inclined plane rigidly mounted to said housing and block means positioned on said inclined plane and slidable relative to said inclined plane and housing, said block means including means for suspending therefrom a cable with which weights are to be lifted.

8. A load limit switch as set forth in claim 7 and including means for adjusting said spring means for varying the opposing force imparted to said slidable means.

9. A load limit switch comprising: a housing; first block means mounted in and rigidly connected to said housing, said block means formed with a plane top surface inclined at an angle with respect to the horizontal when said switch housing is in operating position; second block means mounted in said housing and slidable relative to the housing, said second block means positioned to rest on the first block means, said second block means formed with a plane bottom surface complementary to the top surface of the first block means and slidable relative thereto; means for connectingto said second block means a cable on which weights are to be suspended; spring biased means positioned between said housing and second block means and formed to bias with a predetermined force the second block means in a predetermined reference position relative to the housing and first block means; and switch means connected to said housing and activated by said second block means upon translation thereof from said predetermined reference position a preselected distance along the inclined top surface of the first block means under the force of weight above a predetermined amount suspended from a cable connected to said second block means.

Ml. A load limit switch as set forth in claim 9 wherein is provided means for adjusting said spring bias means for varying the predetermined force for biasing said second block means. 

1. A load limit switch comprising: a housing; an inclined plane rigidly mounted in said housing; block means positioned on and slidable relative to said inclined plane and housing, said block means including means for suspending therefrom a cable for supporting weight; resilient bias means positioned between the housing and block means for biasing said block means with a preselected force at a position on the inclined plane; and switch means activated by said block means upon translation of the block means a predetermined distance along the inclined plane against said spring bias means under the force of weight above a predetermined amount supported on a cable suspended from said block means.
 2. A load limit switch as set forth in claim 1 and including means for adjusting the resilient bias means for bearing said preselected force imparted to the block means.
 3. A load limit switch as set forth in claim 1 wherein said switch means comprises an electric switch actuated by a pushbutton, said electric switch mounted in the housing at a position so that the pushbutton is depressed by the block means when the block means is at said reference position on the inclined plane.
 4. A load limit switch as set forth in claim 1 wherein said switch means comprises an electric switch actuated by a pushbutton, said electric switch mounted in the housing in a position so that the pushbutton is depressed by said block means upon translation of said block means a predetermined distance along the inclined plane under the force of weight above a predetermined amount supported on a cable suspended by said block means.
 5. A load limit switch for interposition between lifting machinery and a suspended cable from which weights are to be supported comprising: a housing with means for connection to said lifting machinery; an inclined plane rigidly mounted in said housing; block means positioned on and slidable relative to said inclined plane and housing, said block means including means for suspending therefrom a cable for supporting weights; spring bias means positioned between the housing and block means for biasing said block means with a preselected force at a reference position on the inclined plane; switch means actuated by said block means upon translation of said block means a predetermined distance against said spring biased means along the inclined plane under the force of weight above a predetermined amount lifted on a cable in turn suspended from said block means; and circuit means controlled by said switch for shutting off the lifting machinery.
 6. A load limit switch as set forth in claim 5 and including means for adjusting the spring bias means for varying the preselected force imparted to said block means.
 7. A load limit switch comprising: a housing; means for suspending from said housing a cable with which weights are to be lifted; means mounted in said housing for directing a fraction of the force on the suspended cable due to lifted weights in a lateral direction within the housing; means in said housing slidable in response to said laterally directed fractional force; spring means mounted in said housing and biased to oppose with a predetermined opposing force motion of said slidable means due to said laterally directed fractional force; and switch means activated by said slidable means when said laterally directed fractional force exceeds said predetermined opposing force; said means mounted in said housing for laterAlly directing a fraction of the force on the suspended cable due to lifted weight comprising an inclined plane rigidly mounted to said housing and block means positioned on said inclined plane and slidable relative to said inclined plane and housing, said block means including means for suspending therefrom a cable with which weights are to be lifted.
 8. A load limit switch as set forth in claim 7 and including means for adjusting said spring means for varying the opposing force imparted to said slidable means.
 9. A load limit switch comprising: a housing; first block means mounted in and rigidly connected to said housing, said block means formed with a plane top surface inclined at an angle with respect to the horizontal when said switch housing is in operating position; second block means mounted in said housing and slidable relative to the housing, said second block means positioned to rest on the first block means, said second block means formed with a plane bottom surface complementary to the top surface of the first block means and slidable relative thereto; means for connecting to said second block means a cable on which weights are to be suspended; spring biased means positioned between said housing and second block means and formed to bias with a predetermined force the second block means in a predetermined reference position relative to the housing and first block means; and switch means connected to said housing and activated by said second block means upon translation thereof from said predetermined reference position a preselected distance along the inclined top surface of the first block means under the force of weight above a predetermined amount suspended from a cable connected to said second block means.
 10. A load limit switch as set forth in claim 9 wherein is provided means for adjusting said spring bias means for varying the predetermined force for biasing said second block means. 